Nanoscale adhesion and sliding on biased semiconductors

We present experimental and theoretical results on controlling nanoscale sliding friction and adhesion by electric fields on model contacts realized by bringing a conductive atomic force microscope tip into contact with the surface of a silicon-oxide/silicon wafer. We find that applying a bias voltage on silicon (or on the conductive tip) enables a noticeable control of the sliding forces. Two electrostatic interactions are identified as being relevant for the friction variation as a function of applied voltage. The first is a short-range electrostatic interaction between opposite charges localized at oxide-silicon/silicon and tip/silicon-oxide interfaces. This attractive interaction results from the high capacity of the oxide-semiconductor interface to change its charge density in response to a bias voltage. Various regimes of charging resulting from silicon electronic bands' alignment and deformation are evidenced. We mainly focused here on the strong charge accumulation and inversion domains. The second longer-range electrostatic interaction is between the voltage-induced bulk and surface charges of both tip and sample. This interaction decreases very slowly with the distance between tip and silicon surface, i.e. oxide thickness, and can be attractive or repulsive depending on voltage polarity. Our results demonstrate the possibility of controlling nanoscale friction/adhesion in nanoscale contacts involving semiconductors. These results are relevant for the operation of nanoscale devices or for on-surface nanomanipulation of metallic nanoparticles. We model the experimental results by adding an electrostatic energy contribution to the tip-surface binding energy, which translates into an increase or decrease of the normal force and ultimately of the sliding friction.